Japanese Patent Kokai Publication (unexamined) No. 78223/1984 discloses a novel polymer prepared by polymerizing an ethylenically unsaturated compound in the presence of an organic polymer having at least one reactive silicone group in a molecule. This polymer has superior properties to those of the organic polymer having at least one reactive silicone group in a molecule. Particularly, the polymer the backbone of which is polyether has significantly improved weather resistance. However, the tensile properties of the cured material of the polymer are not satisfactorily improved. For example, elongation at break of the cured polymer is inferior to that of the organic polymer having at least one reactive silicone group in a molecule. Inferior elongation restricts application fields of the polymer and, for example, it cannot be practically used as a sealing material, which is one of important application fields of the organic polymer having at least one reactive silicone group in a molecule.